The FTSE 100 lost 12.2% on Black Monday, October 19, 1987.
London's market was among the international exchanges hit after the Dow Jones Industrial Average plunged in New York. The FTSE 100's fall reflected the speed with which modern financial markets transmitted fear across borders and time zones.
Black Monday was not confined to the United States. Major markets in Asia, Europe, and North America recorded severe losses, although the exact percentage and timing differed from exchange to exchange. The FTSE 100's 12.2% decline made it one of the most dramatic sessions in the index's history.
The episode also encouraged regulators and exchanges to strengthen market safeguards. Circuit breakers and trading halts became more prominent after 1987, although their design and use vary by market. The FTSE 100 was launched in 1984, so the crash occurred only a few years after the index began.